This chapter introduces cataloguing the world's endangered languages. It outlines the history of the Catalogue of Endangered Languages project, describes the Catalogue database, and highlights the Catalogue's achievements in producing new knowledge, filling gaps in information, correcting errors, and updating information. The chapter addresses how endangered languages are identified, the causes of language endangerment, and the many reasons why loss of languages matters to us all. Language loss is often experienced as a crisis of social identity. A major goal of linguistics is to understand human cognition and human language capacity through the study of what is possible and impossible in human languages. Discovery of previously unknown linguistic features and traits as we document languages contributes to achieving this goal and advances knowledge of how the human mind works. Conversely, language extinction is horrendous, impeding achievement of this goal.